Display device, method for manufacturing same, polyimide film for display device supporting bases, and method for producing polyimide film for display device supporting bases

ABSTRACT

Provided is a display device that can be made thin, lightweight, and flexible, has no problems of cracks and peeling caused by thermal stress, and is excellent in dimension stability and the like. The display device includes: a supporting base including a polyimide film; and a gas barrier layer formed on the supporting base, in which the polyimide film has a transmittance of 80% or more in a wavelength region of from 440 nm to 780 nm, and a coefficient of thermal expansion of 15 ppm/K or less, and has a difference in coefficient of thermal expansion from the gas barrier layer of 10 ppm/K or less.

TECHNICAL FIELD

The present invention relates to a display device and a manufacturingmethod therefor, and to a polyimide film for a display device supportingbase and a production method therefor. More specifically the presentinvention relates to a display device including a supporting baseincluding a polyimide film and a gas barrier layer formed on thesupporting base and a manufacturing method therefor, and to a polyimidefilm for a display device supporting base and a production methodtherefor.

BACKGROUND ART

An organic EL device to be used for various displays, for example,large-size displays such as a television and small-size displays such asa mobile phone, a personal computer, and a smartphone is generallymanufactured by forming thin film transistors (hereinafter referred toas “TFTs”) on a glass substrate serving as a supporting base,successively forming an electrode, a light-emitting layer, and anelectrode on the glass substrate having the TFTs formed thereon, andfinally hermetically sealing the resultant with a separate glasssubstrate, multi-layered thin film, or the like. As structures of theorganic EL device, there are given a bottom-emission structure in whichlight is extracted from the side of a glass substrate serving as asupporting base and a top-emission structure in which light is extractedfrom the side opposite to a glass substrate serving as a supportingbase, and those structures are used properly depending on theapplications. As another structure of the organic EL device, there maybe adopted a structure that allows outside light to directly passtherethrough, and hence there has also been proposed a transparentstructure in which electronic elements such as TFTs are visible from theoutside. All of the above-mentioned structures can be realized byselecting transparent electrodes and substrate materials.

In addition, the organic EL device can be made thin, lightweight, andflexible by substituting a resin for the related-art glass substrate asthe supporting base of the organic EL device. As a result theapplications of the organic EL device can further expanded. However, theresin is generally inferior to glass in dimension stability,transparency, heat resistance, moisture resistance, gas barrierproperty, and the like, and hence various studies of the resin have beenconducted.

For example, JP 2008-231327 A (Patent Literature 1) relates to theinvention of a polyimide and a precursor thereof useful as a plasticsubstrate for a flexible display and has reported that polyimidesobtained by subjecting tetracarboxylic acids containing an alicyclicstructure such as cyclohexylphenyl tetracarboxylic acid to reactionswith various diamines are excellent in transparency. However there is aproblem in that the polyimides obtained in the foregoing have a glasstransition temperature of up to 337° C. according to Examples (Table 1),and hence cannot withstand a heat treatment temperature generallyreaching about 400° C. during the annealing step of TFTs. In addition,all of the obtained polyimides have a coefficient of thermal expansion(CTE) of from about 50 ppm/K to about 60 ppm/K. Therefore, in the casewhere a gas barrier layer is formed so as to impart gas barrier propertyto the polyimide as in Patent Literature 2 described later, peeling andcracks occur at an interface between the gas barrier layer and thepolyimide, with the result that it is difficult to obtain an organic ELdevice excellent in shape stability.

Further, JP 2011-238355 A (Patent Literature 2) relates to the inventionof a gas barrier film that is excellent in gas barrier property and heatresistance, is flexible, and can be used as a base for an organic Eldevice or the like, and discloses the following: a flexible film made ofpolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polycarbonate (PC), polyvinyl chloride (PVC), polyimide, or the like isused as a base; a stress relaxation layer and a gas barrier layer(inorganic barrier layer) including a compound containing at leastsilicon and oxygen are formed on one surface side of the flexible filmso as to prevent the permeation of water vapor and air; and thecoefficient of thermal expansion of the stress relaxation layer is setwithin a range of from 0. 5 ppm/K to 20 ppm/K so as to prevent theoccurrence of peeling and cracks caused by differences in thermophysicalproperties (coefficient of thermal expansion, thermal shrinkage rate)between the resin base and the inorganic barrier layer. However, thereis a problem in that the flexible film made of PET, PEN, PC, PVC, or thelike given as the supporting base in the foregoing does not havesufficient heat resistance and hence cannot withstand the heat treatmenttemperature generally reaching about 400° C. during the annealing stepof TFTs. Further, polyimide (gas barrier film 2-3) used in ComparativeExample has a transmittance lower than that of glass due to a yellowishbrown color thereof and hence is not preferred as a resin to besubstituted for glass.

Further, JP 2007-46054 A (Patent Literature 3) relates to the inventionof a low tinted polyimide resin composition useful for glass typeapplications in the field of electronic displays, and discloses that apolyimide film containing a perfluoro-imide moiety has a low coefficientof thermal expansion and a high glass transition temperature and isexcellent in transparency. However, the transmittance in a visible lightregion of most of the polyimide films actually obtained in Examples hasnot reached 80%, and the glass transition temperature thereof has notreached 400° C. Thus, a polyimide film that concurrently satisfies lowthermal expandability, transparency, and heat resistance has not beenobtained.

Similarly, JP 2-251564 A (Patent Literature 4) discloses that afluorine-containing polyimide composition containing a fluorinated alkylgroup introduced into an acid anhydride and a diamine has a lowdielectric constant, a low water absorption coefficient, and low thermalexpandability and is applicable to materials for printed boards andoptical waveguides. However, Patent Literature 4 does not describe thetransmittance in a visible light region of a polyimide film. Further,Patent Literature 4 does not describe means for solving the retardationproblem caused when a transparent polyimide film having a lowcoefficient of thermal expansion is applied to a supporting base of adisplay device.

Besides the above, attempts have been made to reduce the weight of adevice by rising a flexible resin for a supporting base. For example,Non Patent Literatures 1 and 2 listed below have proposed organic ELdevices in which polyimide having high transparency is applied to thesupporting base. However, as described above, it cannot be said that thedifference in coefficient of thermal expansion between the polyimidefilms described in those literatures and the gas barrier layer of aninorganic compound formed so as to compensate for gas barrier propertyis sufficiently small.

CITATION LIST Patent Literature

[PTL 1] JP 2008-231327 A

[PTL 2] JP 2011-238355 A

[PTL 3] JP 2007-46054 A

[PTL 4] JP 2-251564 A

Non Patent Literature

[NPL 1] S. An et. Al., “2.8-inch WQVGA Flexible AMOLED Using HighPerformance Low Temperature Polysilicon TFT on Plastic Substrates”, SID10 DIGEST, p706 (2010)

[NPL 2] Oishi et. Al., “Transparent PI for flexible display”, IDW '11FLX2surasshuFMC4-1

SUMMARY OF INVENTION Technical Problem

As described above, the organic EL device has low resistance to water,and the characteristics of an EL element serving as a light-emittinglayer are degraded due to water. Therefore, in the case where a resin isused for the supporting base, it is necessary to form a gas barrierlayer on at least one surface of the supporting base so as to preventthe entry of water and oxygen into the organic EL device. In general, asa gas barrier layer excellent in gas barrier performance, inorganicmaterials typified by silicon oxide and silicon nitride have been used,and those materials generally have a coefficient of thermal expansion(CTE) of from 0 ppm/K to 10 ppm/K. In contrast, transparent polyimidegenerally has a CTE of about 60 ppm/K. Therefore, when an attempt ismade to simply apply the transparent polyimide to the supporting base ofthe organic EL device, for example, there arises a problem in thatcracks and peeling occur in the gas barrier layer due to the thermalstress.

Further, the annealing step during which the temperature reaches about400° C. is required for forming TFTs necessary for the displayapplication. Although there is no particular problem in introducing theannealing step in the case of using a related-art glass substrate, whena resin is used for the supporting base, it is necessary that the resinhave heat resistance and dimension stability at the heat treatmenttemperature of the TFT. On the other hand, there is a case in which theTFTs are not required as in an organic EL device for illumination.However, the power consumption of the organic EL device can be reducedby increasing the film formation temperature of a transparent electrodeadjacent to the supporting base to lower the resistance of thetransparent electrode, and hence the supporting base is also required tohave heat resistance even in the case of the illumination application.Further, metal oxides such as ITO have been generally used for thetransparent electrode and have a CTE of from 0 ppm/K to 10 ppm/K, andhence a resin having a CTE in the similar range is required in order tosolve the problems of cracks and peeling.

In order to perform color display with an organic EL display device,materials capable of emitting light of three primary colors, i.e., red(R), green (G), and blue (B) are respectively deposited from the vaporfor the respective colors through use of a shadow mask. However, thismethod has a problem in that it is very difficult and expensive toproduce the shadow mask. Further, it is difficult to achieve highdefinition and an increase in size in the production of the shadow mask.In order to solve those problems, there has been proposed an organic ELdisplay device that performs color display by a combination of a colorfilter with an organic EL emitting white light.

The color filter has a configuration in which a black matrix and coloredlayers of R, G, B, and other colors are formed on a base such as glassor a transparent film, and heat treatment of a resist at a temperaturegenerally reaching 230° C. or more is required for producing the colorfilter. Further, in order to reduce outgas from the resist that may haveadverse effects on an EL element, heat treatment at a temperature of300° C. or more is performed in some cases. Therefore, in the case wherethe coefficient of thermal expansion and the coefficient of humidityexpansion of the supporting base in a display section including the ELelement are not matched with those of the color filter, a difference iscaused in dimension change between the respective substrates due to thechanges in temperature and humidity, which causes warping and peeling.Thus, it is desired that the coefficient of thermal expansion and thecoefficient of humidity expansion of the supporting base in the displaysection including the EL element be matched with those of the colorfilter, or the supporting base in the display section including the ELelement and the base of the color filter be made of the same material.

The polyimide film is generally colored in yellowish brown. Therefore,there is a problem in that, in the case where minute foreign matter ismixed in the polyimide film, it is difficult to find the foreign matterwith naked eyes or a visual inspection device. In particular, it is verydifficult to find foreign matter such as rust of a metal having a colorclose to that of polyimide. The presence of the foreign matter in thepolyimide film causes defects of a gas barrier layer to be formed on thepolyimide film and failures such as disconnection and short circuitingbetween electrodes. The use of the polyimide film having transparencymakes it easy to find the foreign matter and contributes to theprevention of a decrease in yield. Therefore, even in a display devicesuch as electronic paper that does not require transparency as thefunction of the display device from the supporting base, the use of thepolyimide film having transparency leads to the enhancement ofproductivity.

Similarly to the foreign matter, scratches on the surface of thepolyimide film also cause defects of a gas barrier layer and failuressuch as disconnection and short circuiting between electrodes. In thecase where the polyimide film is applied to the supporting base of adisplay device, a defect of 1 μm or less, which is allowed in a flexibleprinted wiring board that is the current main application of thepolyimide film, presents a problem. In the currently commerciallyavailable polyimide films including general yellowish brown polyimidefilms (Kapton, Apical, Upilex, and the like) as well as the transparentpolyimide film, those having a surface state applicable to thesupporting base of a display device without any problems cannot befound.

Further, the transmittance in a visible light region of glass isgenerally about 90%, and in the case where a resin is used for thesupporting base, it is necessary that the transmittance of the resinneed to be set as close as possible to about 90%. The wavelength oflight emitted from the light-emitting layer of the organic EL is mainlyfrom 440 nm to 780 nm, and hence the supporting base used in the organicEL device is required to have an average transmittance of at least 80%in this wavelength region. In addition, it is desired that the resinitself forming the supporting base have moisture resistance.

There is a case in which, when the retardation in an in-plane directionof the supporting base is more than 10 nm, viewing angle characteristicsof uniform contract may not be obtained. In the case where outside lightenters the organic EL device, the electrodes reflect the outside light,with the result that the contrast decreases. In this case, there is amethod of preventing the reflection of the outside light with acircularly polarizing plate. However, when the retardation is large, theeffect of preventing the reflection of the outside light decreases.Thus, in order to obtain high contrast, it is appropriate that theretardation be as small as possible.

It has been known to obtain a polyimide film having a low coefficient ofthermal expansion by stretching a film with a tenter so as to orientmolecular chains. However, there is a problem in that the orientation ofthe molecular chains becomes non-uniform due to the variation in stressapplied to the film during the stretching, with the result thatanisotropy is caused in a refractive index so as to increase theretardation.

It has also been known to obtain a polyimide film having a lowcoefficient of thermal expansion without stretching the polyimide filmby forming polyimide having a rigid chemical structure into a film undersuitable conditions of heat treatment, film thickness, solvent type, andthe like. However, there is a problem in that the molecular chains ofthe polyimide having a rigid chemical structure are easily oriented, andhence the orientation of the molecular chains becomes non-uniform due tothe in-plane variation in temperature during heat treatment and filmthickness so as to cause anisotropy in a refractive index, with theresult that the retardation increases.

Specifically, it is necessary to use a resin capable of concurrentlysatisfying at least a low CTE, heat resistance, and transparency forsubstituting a supporting base of a resin film for a glass substratethat has been hitherto used in the display device. However, the resinfilm for the supporting substrate of the display device capable ofsatisfying all the conditions has not existed. Further, in particular,it is important to control the physical property values at an interfacebetween the resin film and the gas barrier layer in view of thespecialty of the manufacturing process for the display device. Then, asa result of the repeated earnest studies by the inventors of the presentinvention, the inventors of the present invention have found that adisplay device excellent in dimension stability is obtained by formingpolyimide containing a predetermined repeated structure into a polyimidefilm under specified production conditions and setting the difference incoefficient of thermal expansion between the polyimide and a gas barrierlayer to 10 ppm/K or less. Thus, the present invention has beencompleted.

Thus, it is an object of the present invention to provide a displaydevice, such as an organic EL display, an organic EL illuminator,electronic paper, and a liquid crystal display, which can be made thin,lightweight, and flexible, has no problems of cracks and peeling causedby thermal stress, is excellent in dimension stability, and can preventa trouble in a manufacturing process so as to exhibit long-life andsatisfactory element characteristics, and a manufacturing methodtherefor.

Further, it is another object of the present invention to provide apolyimide for a display device supporting base, which can be made thin,lightweight, and flexible and can exhibit long-life and satisfactorycharacteristics, and a production method therefore. As used herein, thedisplay device supporting base refers to a supporting base for formingthe above-mentioned display device, the supporting base having aconfiguration in which any one or two or more of thin film transistors,an electrode layer, an organic EL light-emitting layer, electronic ink,and a color filter are formed on a polyimide film.

Solution to Problem

That is, according to one embodiment of the present invention, there isprovided a display device, including: a supporting base including apolyimide film; and a gas barrier layer formed on the supporting base,in which the polyimide film has a transmittance of 80% or more in awavelength region of from 440 nm to 780 nm, and a coefficient of thermalexpansion of 15 ppm/K or less, and has a difference in coefficient ofthermal expansion from the gas barrier layer of 10 ppm/K or less.

Further, according to one embodiment of the present invention, there isprovided a polyimide film for a display device supporting base, which isused as a supporting base for forming a display device, the polyimidefilm having a transmittance of 80% or more in a wavelength region offrom 440 nm to 780 nm and a coefficient of thermal expansion of 15 ppm/Kor less.

Further, according to one embodiment of the present invention, there isprovided a manufacturing method for a display device, the manufacturingmethod including: applying a resin solution of polyimide or a polyimideprecursor onto a base substrate so that a thickness of a polyimide filmbecomes 50 μm or less; forming the polyimide film on the base substrateby completing heat treatment forming a stress relaxation layer on thepolyimide film; removing the base substrate under a state in which thepolyimide film and the stress relaxation layer are laminated, andsuccessively forming members for a display device, the polyimide filmincluding a single polyimide layer or a plurality of polyimide layersincluding a main polyimide layer containing polyimide having 70 mol % ormore of a structural unit represented by the following general formula(1).

General Formula

-   [In the formula (1), Ar₁ represents a tetravalent organic group    having an aromatic ring, and Ar₂ represents a divalent organic group    represented by the following general formula (1) or (3).

General Formula

-   [In the general formula (2) or the general formula (3), R₁ to R₈    each independently represent a hydrogen atom, a fluorine atom, an    alkyl group or alkoxy group having 1 to 5 carbon atoms, or a    fluorine-substituted hydrocarbon group, and at least one of R₁ to R₄    in the general formula (2) and at least one of R₁ to R₈ in the    general formula (3) each represent a fluorine atom or a    fluorine-substituted hydrocarbon group.]]

In addition, according to one embodiment of the present invention, thereis provided a production method for a polyimide film for a displaydevice supporting base, the production method including: applying aresin solution of polyimide or a polyimide precursor onto a basesubstrate so that a thickness of a polyimide film becomes 50 μm or less;forming the polyimide film on the base substrate by completing heattreatment; forming a stress relaxation layer on the polyimide film; andremoving the base substrate under a state in which the polyimide filmand the stress relaxation layer are laminated, the polyimide filmincluding a single polyimide layer or a plurality of polyimide layersincluding a main polyimide layer containing polyimide having 70 mol % ormore of a structural unit represented by the following general formula(1).

The polyimide film can be produced by polymerizing a diamine and an acidanhydride serving as raw materials in the presence of a solvent so as toobtain a polyimide precursor resin, and imidizing the polyimideprecursor resin by heat treatment. The molecular weight of the polyimideresin can be mainly controlled by changing the molar ratio between thediamine and the acid anhydride serving as the raw materials, and themolar ratio is generally 1:1. Examples of the solvent includedimethylacetamide, dimethylformamide, n-methylpyrrolidinone, 2-butanone,diglyme, and xylene. One kind of the solvents may be used alone, or twoor more kinds thereof may be used in combination.

Each of the diamine and the acid dianhydride serving as the rawmaterials for the polyimide film used in the present invention may beformed of a single kind of monomer or a plurality of kinds of monomers.The polyimide film of the present invention is preferably formed ofpolyimide having a structural unit represented by the following generalformula (1). Alternatively, the polyimide film of the present inventionis preferably formed of a copolymer using a plurality of kinds ofmonomers each having the structural unit represented by the followinggeneral formula (1), more preferably formed of a polyimide resincontaining 70 mol % or more, preferably 90 mol % to 100 mol % of thestructural unit represented by the general formula (1).

General Formula

-   [In the formula (1), Ar₁ represents a tetravalent organic group    having an aromatic ring, and Ar₂ represents a divalent organic group    represented by the following general formula (2) or (3).

General Formula

-   [To the general formula (2) or the general formula (3), R₁ to R₈    each independently represent a hydrogen atom, a fluorine atom, an    alkyl group or alkoxy group having 1 to 5 carbon atoms, or a    fluorine-substituted hydrocarbon group, and at least one of R₁ to R₄    in the general formula (2) and at least one of R₁ to R₈ in the    general formula (3) each represent a fluorine atom or a    fluorine-substituted hydrocarbon group.]]

Other polyimide resins that may be added in addition to the polyimideresin containing the structural unit represented by the general formula(1) can be selected from general acid anhydrides and diamines. It isdesired that the acid anhydride and the diamine be selected so that thecoefficient of thermal expansion does not become more than 15 ppm/K, andas needed, the acid anhydride and the diamine be adjusted for thicknessand be multi-layered. It is desired that the addition amount of the acidanhydride and the diamine be set to up to 30 mol % or less, morepreferably 10 mol % or less. As an acid anhydride that may be preferablyused as the acid anhydride that satisfies such condition, there aregiven, for example, pyromellitic dianhydride,3,3′,4,4′-biphenyltetracarboxylic dianhydride,1,4-cyclohexanedicarboxylic acid, 1,2,3,4-cyclobutanetetracarboxylicdianhydride, and 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropanedianhydride. As a diamine that may be preferably used as the diaminethat satisfies such condition, there are given, for example, 4,4′-diaminodiphenyl sulfone, trans-1,4-diaminocyclohexane,4,4′-diaminocyclohexylmethane,2,2′-bis(4-aminocyclohexyl)-hexafluoropropane, and2,2′-bis(trifluoromethyl)-4,4-diaminobicyclohexane.

As described above, it is preferred that the polyimide film of thepresent invention contain a fluorine atom or a fluorine-substitutedhydrocarbon group in a part of the chemical structure. For this purpose,the fluorine atom or the fluorine-substituted hydrocarbon group may becontained in Ar₁ or Ar₂ or in both Ar₁ and Ar₂ in the general formula(1). In a more preferred embodiment, it is appropriate that at least oneof R₁ to R₄ in the general formula (2) represent a fluorine atom or afluorine-substituted hydrocarbon group, and at least one of R₁ to R₈ inthe general formula (3) represent a fluorine atom or afluorine-substituted hydrocarbon group.

Specific preferred examples of R₁ to R₈ include —H, —CH₃, —OCH₃, —F, and—CF₃, and it is more preferred that at least one of R₁ to R₈ representany one of —F and —CF₃.

Further, specific examples of R₁ in the general formula (1) include thefollowing tetravalent acid anhydride residues.

Further, specific examples of the diamine residue that provides Ar₂ inthe general formula (1) include the following.

A particularly preferred construction of the polyimide for forming thepolyimide film to be used in the present invention is polyimide formedof structural units represented by the following formulae (4) and (5).In this case, the ratio between the content of the structural unitrepresented by the formula (4) and the content of the structural unitrepresented by the formula (5) in the polyimide is, in terms of molarratio, “(4): (5)”=50:50 to 100:0, preferably “(4): (5)”=70:30 to 95:5,more preferably “(4): (5)”=85:15 to 95:5. The content of thosestructural units in the polyimide is from 90 mol % to 100 mol %.

In this case, the structural unit represented by the general formula (4)is mainly effective for enhancing the properties such as low thermalexpandability and high heat resistance, and the structural unitrepresented by the general formula (5) is mainly effective for enhancinghigh transparency. The polyimide film according to such a preferredembodiment does not exclude including structural units other than thestructural units “a” and “b” represented by the general formulae (4) and(5). Note that, the polyimide film preferably includes the structuralunits other than the structural units “a” and “b” within a range of lessthan 10% in terms of molar ratio and most preferably includes only thestructural units “a” and “b”.

In the above-mentioned description, the polyimide film of the presentinvention may include the structural units other than those representedby the formulae (4) and (5) within a range of less than 10 mol %. Thediamine and the acid anhydride serving as the raw materials used in thepresent invention are not particularly limited, and one kind of diamineand acid anhydride appropriately selected from known diamines and acidanhydrides may be used alone, or two or more kinds thereof may be usedin combination.

The polyimide film can be produced by polymerizing the diamine and theacid anhydride serving as the raw materials in the presence of thesolvent so as to obtain a polyimide precursor resin and imidizing thepolyimide precursor resin by heat treatment. The molecular weight of thepolyimide resin can be mainly controlled by changing the molar ratiobetween the diamine and the acid anhydride serving as the raw materials,and the molar ratio is generally 1:1.

The production method involves first dissolving a diamine in an organicsolvent and adding an acid dianhydride to the solution thus obtained soas to produce polyamide acid serving as a polyimide precursor. Examplesof the organic solvent include dimethylacetamide, dimethylformamide,n-methylpyrrolidinone, 2-butanone, diglyme, and xylene. One kind ofthose solvents may be used alone, or two or more kind thereof may beused in combination. The subsequent imidization step can also beconducted through use of chemical imidization using a condensation agentsuch as acetic anhydride, besides thermal imidization utilizing thermaldehydration described in the following production method for a polyimidefilm.

As the production method for a polyimide film, the following method isexcellent in productivity and has been most widely performedindustrially. The method involves casting and applying a resin solutionof polyamide acid or polyimide serving as a raw material for a polyimidefilm onto a base substrate such as a metal roll, drying the resinsolution by heating on the base substrate so as to obtain a gel filmhaving self-supporting property, peeling the gel film from the basesubstrate, and further heating the gel film at high temperature whileholding the gel film with a tenter or the like so as to obtain apolyimide film. However, in this method, the gel film is stretched dueto the stress applied thereto during the peeling of the gel film fromthe base substrate and the tension of the tenter during the heattreatment, with the result that the retardation increases. Therefore,this method is not preferred as the production method for a polyimidefilm of the present invention.

As the production method for a polyimide film of the present invention,for example, the following method is preferred. The method involvescasting and applying a resin solution of polyamide acid onto any basesubstrate made of a copper foil with an applicator, predrying the resinsolution, further removing a solvent from the resin solution, subjectingthe resultant to heat treatment so as to imidize the resultant, andremoving the base substrate used during the imidization by peeling,etching, or the like. When the resin solution is cast and applied ontothe base substrate, it is preferred that the viscosity of the resinsolution be set within a range of from 500 cps to 70,000 cps. Further,the resin solution may be applied onto the base substrate serving as anapplication surface for the resin solution after the surface of the basesubstrate is appropriately subjected to surface treatment. In theabove-mentioned description, it is appropriate that the drying conditionbe from 2 minutes to 30 minutes at 150° C. or less, and the heattreatment for imidization be performed at a temperature of from about130° C. to about 360° C. for from 2 minutes to 30 minutes.

In order to decrease the retardation in an in-plane direction in theabove-mentioned production method for a polyimide film involvingapplying the resin solution of polyamide acid onto the base substrateand removing the polyimide film from the base substrate after thecompletion of the heat treatment, it is appropriate that the in-planevariation in film temperature during the heat treatment be reduced. Thein-plane variation in film temperature during the heat treatment ispreferably 6° C. or less, more preferably 2° C. or less.

In order to reduce the in-plane variation in film temperature, it isappropriate that a laminate of the polyamide acid resin and the basesubstrate be subjected to heat treatment in a forced convection typeoven that has been left for a sufficient period of time after reaching apredetermined temperature and has reached a uniform furnace temperature.Further, when the laminate of the resin and the supporting base isbrought into direct contact with an inner surface of the furnace and ashelf plate during heating, a local variation in temperature may occur,and hence it is preferred that the laminate be set so as to be preventedfrom being brought into contact with the inner surface of the furnaceand the shelf plate as much as possible. Further, the laminate of thepolyamide acid resin and the base substrate may be preheated before theheat treatment.

It is not preferred that the thickness of the base substrate be largebecause the heat capacity increases and the resin is not heatedsufficiently from the base substrate side, which causes the variation intemperature in the plane of the film. The thickness of the supportingbase is preferably 3 mm or less, more preferably 0.8 mm or less.Further, in order to reduce the variation in temperature, a metal havinga high heat conductivity may be used for the base substrate.

Further, in order to decrease the retardation in the in-plane direction,it is preferred that the in-plane variation in the thickness of the filmbe reduced. The in-plane variation in the thickness of the polyimidefilm after the completion of the heat treatment is preferably 1/10 orless, more preferably 1/20 or less of the film thickness.

The above-mentioned application method is not particularly limited. Aslong as predetermined thickness accuracy is obtained, known methods suchas a spin coater, a spray coater, a bar coater, or an extrusion methodusing a slit-shaped nozzle can be applied. In general, it has been knownthat, in the case where a solution of a resin having high orientationcontaining rigid molecular chains is applied to a substrate, theretardation occurs due to the shear stress generated during theapplication. Surprisingly, the application method does not influence theretardation in the present invention. Therefore, any application methodsatisfying both the film thickness accuracy and the productivity can beselected.

Due to the above-mentioned heat treatment, a polyimide film having smallretardation in the in-plane direction while keeping a low coefficient ofthermal expansion and having a transmittance of 80% or more in awavelength region of from 440 nm to 780 nm is obtained on the basesubstrate. In the present invention, it is preferred that the heatingtime, in particular, in a high heating temperature range from atemperature lower by 20° C. than the highest heating temperature(highest attained temperature) to the highest attained temperatureduring the increase in temperature in the above-mentioned heat treatment(hereinafter referred to as “high-temperature retention time”) be setwithin 15 minutes. When the high-temperature retention time exceeds 15minutes, the transparency of the polyimide film tends to be degraded dueto the coloring and the like. In order to maintain the transparency, itis appropriate that the high-temperature retention time be shorter, butthere is a risk in that the effect of the heat treatment may not besufficiently obtained when the time is too short. The optimumhigh-temperature retention time is preferably set to 0.5 minute or moreand 5 minutes or less although the optimum high-temperature retentiontime varies depending on the heating method, the heat capacity of thebase substrate, the thickness of the polyimide film, and the like.

In the case of removing the base substrate by etching in theabove-mentioned production method for a polyimide film involvingapplying a resin solution of polyamide acid onto the base substrate andremoving the polyimide film from the base substrate after the completionof the heat treatment, the following steps are general performed: thebase substrate is removed from the polyimide film by etching; thepolyimide film is washed with running water; water droplets on thesurface of the polyimide film are removed with an air knife; and thepolyimide film is dried by heating in an oven. When the polyimide filmis stretched with the stress generated with respect to the polyimidefilm during those steps, the retardation in the in-plane directionincreases. This tendency is particularly conspicuous in a polyimide filmhaving a low coefficient of thermal expansion because the polyimide filmhas rigid molecular chains. Therefore, it is preferred to reduce thestress applied to the film in the plane direction after a singlepolyimide film is obtained by etching the base.

In order to prevent the stretching of the polyimide film in a series ofprocesses involving etching, the following method maybe used. The methodinvolves forming a stress relaxation layer on a polyimide film andetching a base substrate under a state in which the polyimide film andthe stress relaxation layer are laminated so that the stress generatedduring the process is dispersed to the polyimide film and the stressrelaxation layer. Although the method of forming the stress relaxationlayer is not particularly limited, for example, the stress relaxationlayer can be formed by a method involving bonding a resin film or metalfoil having a coefficient of thermal expansion suitable as the stressrelaxation layer to a polyimide firm with a pressure-sensitive adhesive,followed by application, vapor deposition, and sputtering.

In the case of removing the base substrate by peeling in theabove-mentioned production method for a polyimide film involvingapplying a resin solution of polyamide acid onto the base substrate andremoving the polyimide film from the base substrate after the completionof the heat treatment, when the polyimide film is stretched in the planedirection due to the stress applied to the polyimide film during thepeeling, the retardation in the in-plane direction increases. Therefore,it is preferred to perform the peeling so that the stress to be appliedto the polyimide film in the plane direction during the peeling issmall.

In order to prevent the stretching of the polyimide film during thepeeling of the polyimide film from the base substrate, the followingmethod may be used. The method involves forming a stress relaxationlayer on a polyimide film and peeling the polyimide film from the basesubstrate under a state in which the polyimide film and the stressrelaxation layer are laminated so that the stress required for thepeeling is dispersed to the polyimide film and the stress relaxationlayer.

The above-mentioned stress relaxation layer may be formed directly onthe polyimide film or may be formed on the polyimide film includingfunctional layers such as an electrode layer, a light-emitting layer,thin film transistors, a wiring layer, and a barrier layer formedthereon.

The stress relaxation layer may serve as a member for forming thedisplay device while being laminated on the polyimide film without beingseparated therefrom after the removal of the polyimide film from thebase substrate. Examples of the member for forming the display deviceinclude a display section such as an organic EL light-emitting layer orelectronic paper, an adhesive, a pressure-sensitive adhesive, a barrierfilm, a protective film, and a color filter. Herein, in the case of thedisplay device including a color filter, a color filter layer formed ofa black matrix and colored sections of R, G, B, and the like may beformed on the polyimide film before the polyimide film is peeled fromthe base substrate so that the color filter layer serves as a stressrelaxation layer.

Further, in order to facilitate the peeling of the polyimide film fromthe base substrate so as to prevent the stretching of the polyimidefilm, the following method may be used. The method involves fixing apolyimide film to another substrate, peeling the polyimide film fixed tothe substrate from a base substrate while preventing the stretching ofthe polyimide film in the plane direction, and then separating thepolyimide film from the substrate. The method of fixings polyimide filmto a base maybe a method involving preparing a base having fine holesextending from an inside of the base to a surface thereof, reducing thepressure inside the base, peeling the polyimide film from a basesubstrate while keeping the polyimide film fixed to the surface of thebase through use of the vacuum, returning the reduced pressure insidethe base to ambient pressure, and separating the polyimide film from thebase. The above-mentioned base may be a resin or a metal such asstainless steel. The surface of the base on the polyimide film side maybe curved.

In order to prevent the stretching of the polyimide film during thepeeling of the polyimide film from the base substrate, other knownmethods can also be applied, in JP 2007-512568 A, there is a disclosurethat a yellow film of polyimide or the like is formed on glass, thinfilm electronic elements are formed on the yellow film, and a bottomsurface of the yellow film is irradiated with UV laser light through theglass so that the glass and the yellow film are peeled from each other.This method is one of the preferred methods as a peeling process of thepresent invention because the polyimide film is separated from the glasswith UV laser light so that the stress is not generated during thepeeling. However, there is also a disclosure that, unlike the yellowfilm, a transparent plastic does not absorb UV laser light, and hence itis necessary to form an absorption/peeling layer of amorphous siliconunder the film in advance.

JP 2012-511173 A discloses that, in order to peel the glass and thepolyimide film from each other through the irradiation with UV laserlight, it is necessary to use a laser having a spectrum within a rangeof from 300 nm to 410 nm.

The wavelength of light emitted from an organic EL light-emitting layeris mainly from 440 nm to 780 nm, and hence the supporting base used inthe organic EL device is required to have an average transmittance of atleast 80% or more in the above-mentioned wavelength region. On the otherhand, in the case of peeling the glass and the polyimide film from eachother through the irradiation with UV laser light as described above,when the transmittance of the supporting base at the wavelength of theUV laser light is high, it is necessary to form an absorption/peelinglayer under the film, which decreases the productivity. In order toperform the peeling without forming the absorption/peeling layer, thepolyimide film itself needs to absorb laser light. Therefore, thetransmittance of the polyimide film at 400 nm is preferably 80% or less,more preferably 60% or less, still more preferably 40% or less. Thus,the peeling can be performed through the irradiation with UV laser lightwithout forming the absorption/peeling layer in spite of the fact thatthe polyimide film is transparent.

Further, in the present invention, the polyimide film has a thicknesswithin a range of preferably from 1 μm to 50 μm, more preferably from 3μm to 4 0 μm, particularly preferably from 5 μm to 30 μm. When thethickness of the polyimide film is less than 1 μm, it is difficult tocontrol the thickness with an applicator, with the result that thethickness is liable to become non-uniform. In contrast, when thethickness of the polyimide film is more than 50 μm, there is a risk inthat the heat resistance and the light transmittance may be degraded.

In this case, from the viewpoint of controlling the film thickness to beuniform during the application using an applicator or the like, it ispreferred that the polymerization degree of the polyamide acid andpolyimide to be used for forming the polyimide film fall within a rangeof from 500 cP to 200,000 cP in terms of solution viscosity when thepolymerization degree is represented by the viscosity range of thepolyamide acid solution.

The polyimide film serving as the supporting base in the presentinvention may be formed of a plurality of polyimide layers as long asthe polyimide film satisfies a transmittance of 80% or more in awavelength region of from 440 nm to 780 nm, a coefficient of thermalexpansion of 15 ppm/K or less, and a difference in coefficient ofthermal expansion from the gas barrier layer of 10 ppm/K or less. Thatis, the polyimide having the structural unit represented by the generalformula (1) has relatively hard property having a modulus or elasticityof from about 5 GPa to about 10 GPa. Therefore, a polyimide layer havinga modulus of elasticity lower than this range may be arranged so as tobe brought into contact with the gas barrier layer so that the polyimidelayer serves for stress relaxation.

In the case of using a plurality of polyimide layers, it is preferredthat a polyimide layer to be brought into contact with the gas barrierlayer have a modulus of elasticity lower than that of a polyimide layer(main polyimide layer) having a largest thickness in the polyimide film.The polyimide layer to be brought into contact with the gas barrierlayer also serves as a stress relaxation layer for preventing thestretching of the main polyimide layer having a low coefficient ofthermal expansion, and hence the retardation of the polyimide filmhaving low thermal expandability can be further decreased.

In this case, the modulus of elasticity of the polyimide layer to bebrought into contact with the gas barrier layer is preferably less than5 GPa, more preferably 0.1 GPa or more and less than 5 GPa, particularlypreferably 2 GPa or more and less than 5 GPa. The polyimide layer havingsuch a modulus of elasticity can be formed of widely known polyimides,and in general, the transmittance in a visible light region of thosepolyimides is lower than that of the polyimide having the structuralunit represented by the general formula (1), and the CTE thereof isrelatively high. Therefore, it is appropriate that the polyimide layerto be brought into contact with the gas barrier layer have a thicknessof from 0.5 μm to 10 μm, preferably from 1 μm to 5 μm. That is, in thecase of forming the polyimide film of a plurality of polyimide layers,it is preferred that the polyimide layer (main polyimide layer) having alargest thickness in the polyimide film be formed of the structural unitrepresented by the general formula (1), and the polyimide layer having amodulus of elasticity lower than that of the main polyimide layer bearranged on the gas barrier layer side so that the polyimide layer to bebrought into contact with the gas barrier layer has a modulus ofelasticity lower than that of another polyimide layer adjacent to thepolyimide layer to be brought into contact with the gas barrier layer.The thickness ratio between the thickness of the main polyimide layerand the polyimide layer having a low modulus of elasticity (mainpolyimide layer/polyimide layer having a low modulus of elasticity) inthe case of forming the polyimide film of a plurality of polyimidelayers is preferably from 3 to 50, more preferably from 5 to 20.

It is only necessary that the polyimide film used in the presentinvention have a transmittance of 80% or more in a wavelength region offrom 440 nm to 780 nm at a predetermined thickness, and the thicknessrange is not particularly limited. Preferably, it is appropriate that,in the case of forming a polyimide film having a thickness of 25 μm, thepolyimide film be formed of polyimide that imparts a transmittance of80% or more in a wavelength region of from 440 nm to 780 nm, and suchpolyimide is formed of the above-mentioned polyimide. The polyimidesrepresented by the general formulae (4) and (5) are particularlypreferred.

The present invention relates to an organic EL device including asupporting base including a polyimide film, a gas barrier layer formedon the supporting base, and an organic EL light-emitting layer formedabove the gas barrier layer. As described above, in the organic ELdevice using a resin as the supporting base, the gas barrier layer isgenerally formed on at least one surface of the supporting base so as toprevent the entry of water and oxygen into the organic EL light-emittinglayer. In this case, as the gas barrier layer having barrier propertywith respect to oxygen, water vapor, and the like, there are preferablyillustrated films formed of inorganic oxides such as silicon oxide,aluminum oxide, silicon carbide, silicon oxycarbide, siliconcarbonitride, silicon nitride, and silicon oxynitride. In this case,when the difference in CTE between the gas barrier layer of any of theseinorganic oxides and the polyimide film serving as the supporting baseis large, there is a risk in that curling may occur, dimension stabilitymay be degraded, and cracks may occur during the later productionprocess for TFTs. Further, in general, there is a problem of warping inthe case of producing a film having a large area. However, the polyimidefilm of the present invention has a small difference in CTE from the gasbarrier layer, and hence such trouble can be solved. Note that, Table 1shows typical inorganic films for forming the gas barrier layer andcoefficients of thermal expansion thereof. In this case, the coefficientof thermal expansion varies depending on the production method even whenthe composition is the same, and hence the values shown in Table 1 aremerely guidelines. Further, the gas barrier layer may be formed of onekind of the above-mentioned inorganic films or may be formed so as toinclude two or more kinds thereof.

TABLE 1 Coefficient of thermal expansion Name of substance Composition(ppm/K) Silicon carbide SiC_(x) 4.0 to 6.5 Silicon oxide SiO_(x) 0.5 to5.0 Silicon oxycarbide SiOC 10.0 Silicon carbonitride SiCN 4.0 to 6.5Aluminun oxide Al₂O₃ 6.0 to 9.0 Silicon nitride SiN_(x) 2.0 to 5.4Silicon oxynitride SiON 0.5 to 5.4

As is understood from Table 1, the CTE of the materials for forming thegas barrier layer falls within a range of from 0 ppm/K to 10 ppm/K.Therefore, in the case where the CTE of the polyimide film adjacent tothe gas barrier layer is not a value close to this range, warping andthe like may occur. Therefore, the polyimide film of the presentinvention is set to have a coefficient of thermal expansion of 15 ppm/Kor less, preferably from. 0 ppm/K to 10 ppm/K, and a difference incoefficient of thermal expansion from the gas barrier layer of 10 ppm/Kor less, preferably from 0 ppm/K to 5 ppm/K. Note that, in the casewhere the polyimide film is formed of a plurality of polyimide layers,the coefficient of thermal expansion of the entire polyimide film isshown (the same applies to the other characteristics of the polyimidefilm).

Further, the polyimide film of the present invention has a transmittanceof 80% or more, preferably 83% or more in a wavelength region of from440 nm to 780 nm. When the transmittance is less than 80% in thewavelength region, emitted light cannot be extracted sufficiently (inparticular, in the case of the bottom-emission structure). Further, itis appropriate that the polyimide film of the present invention have aheating weight loss of 1.5% or less, preferably 1.3% or less when heldat 460° C. for 90 minutes. When the heating weight loss is more than1.5%, the polyimide film cannot withstand the production processtemperature of the TFTs.

In addition, it is appropriate that the retardation in the in-planedirection of the polyimide film be 10 nm or less, preferably 5 nm orless. When the retardation in the in-plane direction is more than 10 nm,the viewing angle characteristics of uniform contrast may not beobtained. In the case where outside light enters the organic EL device,the electrodes reflect the outside light, with the result that thecontrast decreases. In this case, there is a method of preventing thereflection of the outside light with a circularly polarizing plate.However, when the retardation is large, the effect of preventing thereflection of the outside light is degraded. Thus, in order to obtainhigh contrast, it is appropriate that the retardation be as small aspossible. Further, it is appropriate that the surface roughness Ra ofthe polyimide film be 5 nm or less, preferably 4 nm or less. When thesurface roughness Ra is more than 5 nm, the thickness of the organic ELlayer becomes non-uniform, which causes disconnection, a variation inemission, and a decrease in color reproducibility. Still further, it isappropriate that the polyimide film of the present invention have acoefficient of humidity expansion of 15 ppm/% RH or less, preferablyfrom 0 ppm/% RH to 10 ppm/% RH. When the coefficient of humidityexpansion is more than 15 ppm/% RH, positional displacement caused by achange in dimension during the TFT process and troubles in a reliabilitytest occur.

Advantageous Effects of Invention

The organic EL device of the present invention uses the polyimide filmhaving predetermined characteristics as the supporting base, and hencethe organic EL device having the performance equivalent to that ofrelated-art products can be realized by the manufacturing methodsubstantially equivalent to that for a glass substrate that has beengenerally used hitherto, while being able to be made thin, lightweight,and flexible.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view illustrating an example of anorganic EL device having a bottom-emission structure of the presentinvention.

FIG. 2 is a schematic sectional view illustrating an example of anorganic EL device having a top-emission structure of the presentinvention.

DESCRIPTION OF EMBODIMENTS

The present invention is described in more detail with reference to thedrawings. Note that, in each of figures and Examples, the same orsimilar component members are denoted by the same reference numeral andthe description thereof is omitted.

FIG. 1 is a schematic sectional view illustrating an organic EL devicehaving a bottom-emission structure according to the present invention.Reference numeral 1 in FIG. 1 denotes a supporting base, and in thepresent invention, the supporting base 1 is formed of a polyimide film.A gas barrier layer 3-1 is formed on one surface (principal surface) ofthe supporting base 1. The gas barrier layer 3-1 prevents moisture frompermeating the supporting base 1. Further, a circuit-forming layer 5including thin film transistors (TFTs) (not shown) is formed on an uppersurface of the gas barrier layer 3-1. The circuit-forming layer 5 isconfigured in such a manner that, for example, an anode electrode 6formed of a transparent conductive film of indium tin oxide (ITO) isformed on an upper surface of the circuit-forming layer 5 so as tocorrespond to each of pixel regions arranged in matrix.

Further, a light-emitting layer 7 is formed on an upper surface of theanode electrode 6, and a cathode electrode 8 is formed on an uppersurface of the light-emitting layer 7. The cathode electrode 8 is formedin common to the respective pixel regions. A gas barrier layer 3-2 isformed so as to cover a surface of the cathode electrode 8. Further, asealing substrate 2 for surface protection is arranged on an outermostsurface of the organic EL device. It is preferred that the gas barrierlayer 3-3 be formed on a surface of the sealing substrate 2 on thecathode electrode 8 side. Further, it is desired that the sealingsubstrate 2 be bonded onto the cathode electrode 8 with an adhesive(adhesive layer) 9 containing a desiccant. Thus, the organic EL devicegenerally has a configuration in which the respective thin films areformed on the supporting base 1 in the stated order, and the thin filmsthus formed are sealed with the sealing substrate 2 finally. In general,the sealing substrate 2 is bonded onto the cathode electrode 8 with anadhesive containing a water-absorbing material.

In this case, thin film transistors having a high mobility are requiredfor driving the organic EL device, and in general, low-temperaturepolysilicon TFTs are used. It is generally considered to be appropriatethat the treatment temperature of the TFTs be 450° C. or more. Further,oxide semiconductor TFTs using IGZO having a relatively high mobility inlow-temperature treatment have been studied. However, it is becomingclear from the recent finding that the high-temperature treatment at400° C. or more is required for increasing the stability of the TFTs.Therefore, it is necessary that the polyimide film serving as thesupporting base withstand the heat treatment step of the TFTs.

Further, the light-emitting layer 7 is formed of a multi-layered film(anode electrode-light-emitting layer 7-cathode electrode) such as ahole injection layer-hole transport layer-light-emitting layer-electrontransport layer. In particular, the light-emitting layer 7 is degradeddue to water and oxygen. Therefore, in general, the light-emitting layer7 is formed by vacuum vapor deposition, and the electrodes and thelight-emitting layer 7 are formed continuously in vacuum.

FIG. 2 is a schematic sectional view illustrating an organic EL devicehaving a top-emission structure. In FIG. 2, the supporting base 1 isformed of a transparent polyimide film. The gas barrier layer 3-1 isformed on one surface (principal surface) of the supporting base 1. Thegas barrier layer 3-1 prevents moisture from permeating the supportingbase 1. The circuit-forming layer 5 including thin film transistors 4(details thereof are not shown) is formed on an upper surface of the gasbarrier layer 3-1. In the case of the top-emission structure, light canalso be extracted from above the thin film transistors 4, and hence theuse efficiency of light increases. The circuit-forming layer 5 isconfigured in such a manner that, for example, a metal thin film servingas a reflection electrode and an indium tin oxide (ITO) thin film foradjusting a work function are formed as the anode electrode 6 on anupper surface of the circuit-forming layer 5 so as to correspond to eachof pixel regions arranged in matrix. The light-emitting layer 7 isformed on an upper surface of the anode electrode 6, and the cathodeelectrode 8 is formed on an upper surface of the light-emitting layer 7.The cathode electrode 8 is formed in common to the respective pixelregions. As the cathode electrode 8, a semi-transparent thin film ofsilver, an alloy thereof, or the like capable of adjusting a workfunction and transmitting a part of light is generally used. In order toreduce the electrode resistance, a transparent electrode of indium sineoxide (IZO) is generally laminated on the semi-transparent thin film.

Further, the gas barrier layer 3-2 is formed so as to cover the surfaceof the cathode electrode 8. Further, the sealing substrate 2 for surfaceprotection is arranged on the outermost surface of the organic ELdevice. It is preferred that the barrier layer 3-3 be formed on thesurface of the sealing substrate 2 on the cathode electrode 8 side. Thesealing substrate 2 is generally bonded onto the cathode electrode 8with the adhesive 9 containing a desiccant. It is necessary that the gasbarrier layer 3-2, the adhesive 9, the gas barrier layer 3-3, and thesealing substrate 2 be transparent.

In the case of the top-emission structure, the supporting base 1 is notnecessarily required to be transparent. However, when the supportingbase 1 is transparent, a TFT pattern and the like can be observed fromthe surface on the supporting base side. Thus, there is obtained anothereffect of the transparent supporting base. On the other hand, thesealing substrate 2 is required to be transparent. If the polyimide filmof the present invention is also used for the sealing substrate 2, thecoefficient of thermal expansion and coefficient of humidity expansionof the supporting base 1 become the same as those of the sealingsubstrate 2, and hence there is obtained an effect that the completedorganic EL device is less liable to foe warped or broken by warping.

Further, the organic EL device of the present invention can also beapplied to organic EL illuminators. In this case, the organic ELilluminators generally have the bottom-emission structure excluding thethin film transistor 4 layer of FIG. 1. However, due to the absence ofthe thin film transistors 4, the anode electrode 6 needs to have lowresistance. In general, a transparent electrode of indium tin oxide(ITO) or the like is used for the anode electrode 6, and the electroderesistance decreases as the transparent electrode is treated at highertemperature. The ITO is generally subjected to heat treatment at from200° C. to 300° C. Note that, the organic EL illuminators tend to beenlarged, and the resistance is becoming insufficient with the ITOelectrode. Thus, various alternative electrode materials have beensearched. In this case, in general, there is a high possibility that thetemperature further higher than the temperature within a range of from200° C. to 300° C. is required, and the polyimide film of the presentinvention can foe preferably used.

EXAMPLES

The present invention is more specifically described below by way ofExamples. However, the present invention is not limited to the scope ofExamples below.

(Formation Method for Polyimide Film Serving as Supporting Base andCharacteristics Thereof)

First, abbreviations of a monomer and a solvent to be used insynthesizing polyimide, and a measurement method of various physicalproperties in the examples and conditions thereof are described below.

TFMB:

-   2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl-   pyromellitic dianhydride-   DMAc; N,N-dimethylacetamide-   6 FDA: 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride-   BPDA: 3,3′,4,440 -biphenyltetracarboxylic dianhydride

“Coefficient of Thermal Expansion (CTE)”

A polyimide film having a size of 3 mm×15 mm was subjected to a tensiletest in a temperature range of from 30° C. to 260° C. at a predeterminedtemperature increase rate (20° C./min) under a load of 5.0 g with athermal mechanical analyzer (TMA) so that a coefficient of thermalexpansion (ppm/K) was measured based on the stretched amount of thepolyimide film with respect to the temperature.

“Transmittance”

A polyimide film (50 mm×50 mm) was measured for an average value oflight transmittance in a wavelength region of from 440 nm to 780 nm witha U4000 spectrophotometer.

“Thermal Weight Loss”

A polyimide film was held in nitrogen at 460° C. for 90 minutes andmeasured for a weight loss before and after heating through use ofTG/DTA7200 manufactured by SII Nano Technology Inc.

“Retardation”

The retardation in an in-plane direction of a polyimide film wasdetermined through use of a spectroscopic polarimeter “Poxi-spectra.”manufactured by Tokyo Instruments, Inc. The measurement was conducted ina wavelength region of from 400 nm to 800 nm. Table 2 shows measuredvalues at a wavelength of 600 nm.

“Surface Roughness”

A surface of a polyimide film that was not held in contact with a basesubstrate during the formation of the film was observed for surfaceroughness Ra in a tapping mode through use of an atomic force microscope(AFM) “Multi Mode 8” manufactured by Broker Japan Co., Ltd. A visualfield measuring 10 μm per side was observed four times, and an averagethereof was determined. The surface roughness (Ra) represents anarithmetic average roughness (JIS B0601-1991).

“Surface Scratches”

An air surface (surface that was not held in contact with a basesubstrate during the formation of a film) of a polyimide film wasobserved for the presence or absence of scratches in a scan-assist modethrough use of an atomic force microscope (AFM) “Multi Mode 8”manufactured by Broker Japan Co., Ltd. A visual field measuring 20 μmper side was observed four times. In the case where no scratches wereobserved, the result was indicated by Symbol “o”. In the case whereminute scratches were observed, the result was indicated by Symbol “Δ”.In the case where streak-like scratched were observed, the result wasindicated by Symbol “x”.

[Coefficient of Humidity Expansion]

A laminate of a copper foil and a polyimide film was cut to a size of 25cm×25 cm. An etching resist layer was formed on the copper foil side,and the resultant was formed into a pattern in which 16 dots each havinga diameter of 1 mm were arranged at an interval of 10 cm on four sidesof a square measuring 30 cm per side. Exposed portions of etching resistopenings were etched to obtain a polyimide film for CHE measurementhaving 16 copper foil remaining points. The polyimide film was dried at120° C. for 2 hours. After that, the polyimide film was left stand stillat each humidity for 24 hours in a constant temperature and humiditydevice having a relative humidity of 30% RH, 50% RH, or 70% RH at 23°C., and a coefficient of humidity expansion (ppm/% RH) was determinedbased on a change in dimension between the copper foil points at eachhumidity measured with a two-dimensional length measuring machine. Thepolyimide film formed on glass was peeled from the glass, and then apattern was marked on the polyimide film so that a coefficient ofhumidity expansion (ppm/% RH) was determined in the same way as theabove.

“Cracks”

A silicon nitride film having a thickness of 50 nm was formed by CVD,and the occurrence of cracks was observed with a microscope “KH-7700”manufactured by Yamato Scientific Co., Ltd. In the case where the numberof cracks was 10 or more in a visual field measuring 10 mm per side, theevaluation result was indicated by Symbol “x”. In the case where thenumber of cracks was 1 or more and less than 9, the evaluation resultwas indicated by Symbol “o”. In the case where no cracks were observed,the evaluation result was indicated by a word “none”.

“Curling”

A silicon oxide film having a thickness of 50 nm was formed as a gasbarrier layer by CVD, and polyimide-gas barrier layer laminatesmeasuring 10 cm per side and 100 cm per side were prepared. Four curlingcorners of each laminate when the laminates were placed on a plane witha convex surface directed downward were visually observed.

Next, the production conditions in Examples are described below.

“Application”

An applicator adjusted so that the in-plane variation in thickness of apolyimide film after heat treatment became 1 μm or less was used.

“Heat Treatment”

In heat treatment using a hot air oven, the heat treatment was startedone hour after the temperature reached a predetermined temperaturethrough use of a forced convection type hot air oven having a blast fan.A laminate of a base substrate and a resin was positioned at the centerof the hot air oven, against which the hot air was most strongly blown.The laminate was set on a table formed of a stainless wire so as not tointervene in the circulation of the hot air, and subjected to the heattreatment. The temperature variation of the laminate at this positionwas 2° C.

In heat treatment using a nitrogen oven, the heat treatment wasconducted by a general method without considering the retardation inparticular. That is, a laminate of a base substrate and a resin was seton an attached shelf plate (stainless punching metal) and subjected tothe heat treatment through use of a nitrogen oven set to a predeterminedtemperature. The temperature variation of the nitrogen oven was 6° C.

Example 1 (Polyimide A)

25.2 g of TFMB were dissolved in a solvent DMAc under stirring in a200-ml separable flask in a nitrogen gas stream. Then, 14.5 g of PMDAand 5.2 g of 6FDA were added to the solution. Then, the solution wascontinuously stirred at room temperature for 5 hours so as to besubjected to a polymerization reaction, and the resultant was kept for awhole day and night. A viscous polyamide acid solution was obtained, andit was confirmed that polyamide acid A having a high polymerizationdegree was generated.

The polyamide acid solution was applied onto a copper foil (electrolyticcopper foil “DFF” manufactured by Mitsui Mining & Smelting Co., Ltd.)having a thickness of 18 μm with an applicator so that the filmthickness after heat treatment became about 25 μm, and the resultant wasincreased in temperature from 90° C. to 360° C. at a rate of 22° C. perminute in a nitrogen oven so as to obtain a laminate of the copper foiland the polyimide. The laminate was immersed in a ferric chlorideetchant so as to remove the copper foil, to thereby obtain film-shapedpolyimide A. Table 2 shows the results obtained by conducting variousevaluations of the film-shaped polyimide A thus obtained. Further, thecurled state of the laminate including a gas barrier layer obtained byforming a silicon oxide film on the polyimide film A was observed, andas a result, no curling was found in the size measuring 10 cm per side.On the other hand, slight curling was found in the size measuring 100 cmper side.

Example 2 (Polyimide B)

25.7 g of TFMB serving as a diamine, 15.7 g of PMDA serving as an acidanhydride, and 3.6 g of 6 FDA were added by having stirred in a 200-mlseparable flask in a nitrogen gas stream. Then, the solution wascontinuously stirred at room temperature for 5 hours so as to besubjected to a polymerization reaction, and the resultant was kept for awhole day and night. A viscous polyamide acid solution was obtained, andit was confirmed that polyamide acid B having a high polymerizationdegree was generated.

Film-shaped polyimide B was obtained in the same way as in Example 1through use of the polyamide acid solution thus obtained.

Example 3 (Polyimide C)

26. 3 g of TFMB were dissolved in a solvent DMAc under stirring in a200-ml separable flask in a nitrogen gas stream. Then, 16.9 g of PMDAand 1.8 g of 6 FDA were added to the solution. Then, the solution wascontinuously stirred at room temperature for 5 hours so as to besubjected to a polymerization reaction, and the resultant was kept for awhole day and night. A viscous polyamide acid solution was obtained, andit was confirmed that polyamide acid C having a high polymerizationdegree was generated.

The polyamide acid solution was applied onto a copper foil (electrolyticcopper foil “DFF” manufactured by Mitsui Mining & Smelting Co., Ltd.)having a thickness of 18 μm with an applicator so that the filmthickness after heat treatment became about 25 μm, and the resultant wasincreased in temperature from 90° C. to 360 at a rate of 22° C. perminute in a hot air oven so as to obtain a laminate of the copper foiland the polyimide film. Next, a pressure-sensitive adhesive film (PETfilm: 100 μm, pressure-sensitive adhesive: 33 μm) was bonded onto asurface of the polyimide film. After that, the laminate was immersed ina ferric chloride etchant so as to remove the copper foil. Further, thepolyimide film was separated from the pressure-sensitive adhesive filmso as to obtain film-shaped polyimide C.

Comparative Example 1 (Polyimide D)

Film-shaped polyimide D was obtained in the same way as in Example 1except for using 23.4 g of TFMB serving as a diamine, 10.3 g of PMDAserving as an acid anhydride, and 11.3 g of 6FDA.

Comparative Example 2 (Polyimide E)

Film-shaped polyimide E was obtained in the same way as in Example 1except for using 23.0 g of TFMB serving as a diamine, 9.3 g of PMDAserving as an acid anhydride, and 12.7 g of 6FDA.

Comparative Example 3 (Polyimide F)

Film-shaped polyimide F was obtained in the same way as in Example 1except for using 23.5 g of TFMB serving as a diamine and 21.5 g of BPDAserving as an acid anhydride.

Example 4

18.9 g of TFMB were dissolved in a solvent DMAc under stirring in a200-ml separable flask in a nitrogen gas stream. Next, 26.1 g of 6FDAwere added to the solution. Then, the solution was continuously stirredat room temperature for 5 hours so as to be subjected to apolymerization reaction, and the resultant was kept for a whole day andnight. A viscous polyamide acid solution G was obtained, and it wasconfirmed that polyamide acid having a high polymerization degree wasgenerated.

Next, the polyamide acid solution C obtained in Example 3 was appliedonto a rolled copper foil having a thickness of 18 μm with an applicatorso that the film thickness after heat treatment became about 25 μm, andthe resultant was heated at a temperature of from 90° C. to 130° C. forfrom 1 minute to 5 minutes in a hot air oven. Then, the polyamide acidsolution G was further applied onto the laminate of the polyamide acidand the copper foil thus obtained so that the thickness became 5 μm, andthe resultant was increased in temperature from 90° C. to 360° C. at arate of 22° C. per minute in a hot air oven so as to obtain a laminateof the copper foil and two layers of the polyimide.

Next, the laminate was immersed in a ferric chloride etchant so as toremove the copper foil, to thereby obtain a polyimide laminate filmformed of the polyimide C and polyimide G. Separately, a single layerfilm of the polyimide G was prepared by the same procedure and measuredfor a modulus of elasticity to be 4.5 GPa.

Example 5

The polyamide acid solution C was applied onto glass having a thicknessof 0.5 mm with an applicator so that the film thickness after heattreatment became about 25 μm, and the resultant was dried by heating at130° C. in a hot air oven so as to remove the solvent in the resinsolution. Then, the resultant was heated at 150° C., 200° C., and 250°C. for 30 minutes and heated at 360° C. for 1 minute so as to obtain alaminate of the glass and the polyimide film. Next, a pressure-sensitiveadhesive film (PET film: 100 μm, pressure-sensitive adhesive: 33 μm) wasbonded onto a surface of the polyimide film, and the polyimide film waspeeled from the glass. Then, the polyimide film was separated from thepressure-sensitive adhesive film so as to obtain film-shaped polyimideC.

Example 6

Film-shaped polyimide C was obtained in the same way as in Example 5except for setting the heating time at 360° C. to 30 minutes.

Example 7

Film-shaped polyimide C was obtained in the same way as in Example 5except for performing the heat treatment in a nitrogen oven.

Example 8

Film-shaped polyimide C was obtained in the same way as in Example 5except for setting the thickness of the glass to 3 mm.

Example 9

Film-shaped polyimide C was obtained in the same way as in Example 5except for performing the application so that the film thickness afterthe heat treatment became about 11 μm.

Example 10

Film-shaped polyimide C was obtained in the same way as in Example 9except for performing the heat treatment in a nitrogen oven.

Example 11

19.2 g of TFMB were dissolved in a solvent DMAc under stirring in a200-ml separable flask in a nitrogen gas stream. Then, 13.1 g of PMDAwere added to the solution. Then, the solution was continuously stirredat room temperature for 5 hours so as to be subjected to apolymerization reaction, and the resultant was kept for a whole day andnight. A viscous polyamide acid solution was obtained, and it wasconfirmed that polyamide acid H having a high polymerization degree wasgenerated,

Film-shaped polyimide H was obtained in the same way as in Example 5except for using the polyamide acid solution H.

Example 12

The polyamide acid solution was applied onto a copper foil (electrolyticcopper foil “DFF” manufactured by Mitsui Mining & Smelting Co., Ltd.)having a thickness of 18 μm with an applicator so that the filmthickness after heat treatment became about 20 μm, and the resultant wasincreased in temperature from 90° C. to 360° C. at a rate of 22° C. perminute in a nitrogen oven so as to obtain a laminate of the copper foiland the polyimide. The laminate was immersed in a ferric chlorideetchant without forming a stress relaxation layer so as to remove thecopper foil, to thereby obtain film-shaped polyimide A.

Example 13

Film-shaped polyimide C was obtained in the same way as in Example 7except for peeling the polyimide film from the glass without using thepressure-sensitive adhesive film.

Example 14

The polyamide acid solution C was applied onto glass having a thicknessof 0.5 mm with an applicator so that the film thickness after heattreatment became about 11 μm, and the resultant was dried by heating at130° C. in a nitrogen oven so as to remove the solvent in the resinsolution, to thereby obtain a laminate of the glass and the gel film.Next, the gel film was peeled from the glass and fixed to a tenter clip.The gel film was heated at 150° C., 200° C., and 250° C. for 30 minutes,and heated at 360° C. for 1 minute so as to obtain polyimide C.

Comparative Example 4

Film-shaped polyimide G was obtained in the same way as in Example 13except for using the polyamide acid solution G.

Comparative Example 5

A commercially available transparent polyimide film (Neopulim Lmanufactured by Mitsubishi Gas Chemical Company, Inc., thickness: 100μm) (hereinafter referred to as “polyimide I”) was measured in the sameway. Both surfaces of the film were observed for surface scratches.

Further, a silicon nitride film having a thickness of 50 nm was formedby CVD on each of the polyimide films A to I and the polyimide laminatefilm of the polyimide films C and G, and the occurrence of cracks wasobserved with a microscope. As a result, no cracks were observed in thelaminate of C/G, and slight cracks were observed in the polyimide filmsA, B, C, and H. Further, a large number of cracks were observed in thepolyimide films D, E, F, G, and I.

Table 2 shows characteristics values of the polyimide films A to I andthe polyimide laminate film (C/G film) of the polyimide films C and Gthus obtained. As shown in Table 2, as is apparent from the resultsobtained from Examples 1 to 14 and Comparative Examples 1 to 5, thepolyimides satisfying the conditions of the present invention were alsoexcellent in transparency and had a low coefficient of thermal expansionwithout any warping, and the surface roughness and the value ofretardation of the surface of each polyimide resin layer were low.Further, curling was hardly confirmed when the gas barrier layer wasformed, and the evaluation on the occurrence of the cracks was alsosatisfactory. On the other hand, the films formed of the polyimide resinlayers not satisfying the conditions of the present invention had alarge coefficient of thermal expansion, and curling was confirmed whenthe gas barrier layer was formed. Further, a large number of cracksoccurred.

TABLE 2 High- temperature heat 400-nm Acid anhydride Diamine treatmentStress Transmit- transmit- PMDA 6FDA BPDA TFMB time relaxation tancetance Polyimide mol % mol % mol % mol % min layer % % Example 1 A 85 150 100 0.9 — 83.4 15.7 Example 2 B 90 10 0 100 0.9 — 82.9 14.1 Example 3C 95 5 0 100 0.9 — 80.5 12.5 Example 4 C/G 0.9 — 80.5 12.1 Example 5 C95 5 0 100 1.0 Present 85.1 11.4 Example 6 C 95 5 0 100 30 Present 80.16.4 Example 7 C 95 5 0 100 1.0 Present 86.1 13.4 Example 8 C 95 5 0 1001.0 Present 85.0 11.4 Example 9 C 95 5 0 100 1.0 Present 86.5 32.0Example 10 C 95 5 0 100 1.0 Present 87.5 34.0 Example 11 H 100 0 0 1001.0 Present 82.1 10.0 Example 12 A 85 15 0 100 0.9 Absent 83.4 15.7Example13 C 95 5 0 100 1.0 Absent 85.1 11.4 Example 14 C 95 5 0 100 1.0Absent 87.6 34.2 Comparative D 65 35 0 100 0.9 — 85.4 24.9 Example 1Comparative E 60 40 0 100 0.9 — 86.1 27.8 Example 2 Comparative F 0 0100 100 0.9 — 76.1 6.1 Example 3 Comparative G 0 100 0 100 1.0 Absent89.8 80.6 Example 4 Comparative I 89.5 83.9 Example 5 CoefficientCoefficient Thermal of of thermal weight Retard- Surface humidityexpansion loss ation roughness Surface expansion ppm/K % nm nm scratchppm/% RH Crack Example 1 9.8 1.3 8 3.2 — 4.8 ∘ Example 2 6.9 1.3 8 4.0 —4.0 ∘ Example 3 3.8 1.4 5 4.0 — 3.5 ∘ Example 4 9.6 1.3 4 4.0 — 3.8 noneExample 5 7.1 1.2 4 2.0 ∘ 3.6 ∘ Example 6 6.2 1.2 4 1.8 ∘ 3.4 ∘ Example7 7.1 1.2 9 1.5 ∘ 3.3 ∘ Example 8 5.0 1.2 8 2.0 ∘ 3.4 ∘ Example 9 1.21.3 4 2.0 ∘ 3.5 ∘ Example 10 1.2 1.3 9 2.0 ∘ 3.5 ∘ Example 11 −1.8 0.8 51.2 ∘ 3.2 ∘ Example 12 9.6 1.3 11 2.8 x 4.8 ∘ Example13 7.1 1.2 15 3.7 Δ3.6 ∘ Example 14 −0.3 1.3 32 2.0 Δ 3.5 ∘ Comparative 16.2 1.1 3 3.8 —4.9 x Example 1 Comparative 20.4 1.3 3 3.7 — 5.0 x Example 2 Comparative63.7 1.1 3 4.8 — 6.4 x Example 3 Comparative 65.7 — 1 — Δ — x Example 4Comparative 52.0 — 47 — x — x Example 5

While the present invention has been described by way of Examples, theconfigurations described in Examples are merely illustrative, andmodifications can be appropriately made without departing from thetechnical concept of the present invention.

REFERENCE SIGNS LIST

1 supporting base

-   2 sealing substrate-   3-1, 3-2, 3-3 gas barrier layer-   4 thin film transistor-   5 circuit-forming layer including thin film transistor-   6 anode electrode-   7 light-emitting layer-   8 cathode electrode-   9 adhesive layer-   LT light to be extracted to outside

1. A display device, comprising: a supporting base comprising apolyimide film; and a gas barrier layer formed on the supporting base,wherein the polyimide film has a transmittance of 80% or more in awavelength region of from 440 nm to 780 nm, a retardation in an in-planedirection of 10 nm or less, and a coefficient of thermal expansion of 15ppm/K or less, and has a difference in coefficient of thermal expansionfrom the gas barrier layer of 10 ppm/K or less.
 2. A display deviceaccording to claim 1, wherein the polyimide film has a surface roughnessRa of 5 nm or less.
 3. A display device according to claim 1 or 2wherein the polyimide film has a heating weight loss of 1.5% or lesswhen held at 460° C. for 90 minutes, and has a coefficient of humidityexpansion of 15 ppm/% RH or less.
 4. A display device according to claim1 or 2, wherein the display device comprises an organic EL devicecomprising: the supporting base comprising the polyimide film; and thegas barrier layer formed on the supporting base, and further comprisingan organic EL light-emitting layer.
 5. A display device according toclaim 4, wherein the organic EL device has a bottom-emission structurein which light is extracted from a surface of the supporting basecomprising the polyimide film through the supporting base.
 6. A displaydevice according to claim 4, wherein the organic EL device has atop-emission structure in which light is extracted from a surface on anopposite side to the supporting base comprising the polyimide film.
 7. Adisplay device according to claim 1 or 2, wherein the display devicecomprises electronic paper comprising: the supporting base comprisingthe polyimide film; and the gas barrier layer formed on the supportingbase, and further comprising an electronic ink layer.
 8. A displaydevice according to claim 1 or 2, further comprising a color filter,wherein the supporting base comprising the polyimide film has a colorfilter layer formed in advance thereon.
 9. A display device according toclaim 1 or 2, wherein the gas barrier layer comprises one kind or two ormore kinds selected from the group consisting of silicon oxide, siliconnitride, silicon oxynitride, silicon carbide, silicon oxycarbide,silicon carbonitride, and aluminum oxide, and the gas barrier layer hasa coefficient of thermal expansion of from 0 ppm/K to 10 ppm/K.
 10. Adisplay device according to claim 1 or 2, wherein the polyimide filmcomprises a plurality of polyimide layers having different modulus ofelasticity, and the polyimide layer to be brought into contact with thegas barrier layer has a modulus of elasticity lower than a modulus ofelasticity of another polyimide layer adjacent to the polyimide layer tobe brought into contact with the gas barrier layer.
 11. A display deviceaccording to claim 10, wherein the polyimide layer to be brought intocontact with the gas barrier layer has a modulus of elasticity of lessthan 5 GPa.
 12. A display device according to claim 1 or 2, wherein thepolyimide film comprises a single polyimide layer or a plurality ofpolyimide layers comprising a main polyimide layer containing polyimidehaving a structural unit represented by the following general formula(4) and a structural unit represented by the following general formula(5) at a molar ratio “(4):(5)” of from 50:50 to 100:0.


13. A polyimide film for a display device supporting base, which is usedas a supporting base for forming a display device, the polyimide filmhaving a transmittance of 80% or more in a wavelength region of from 440nm to 780 nm, a retardation in an in-plane direction of 10 nm or less,and a coefficient of thermal expansion of 15 ppm/K or less.
 14. Apolyimide film for a display device supporting base according to claim13, wherein the polyimide film has a surface roughness Ra of 5 nm orless.
 15. A polyimide film for a display device supporting baseaccording to claim 13 or 14, wherein the polyimide film has a heatingweight loss of 1.5% or less when held at 46° C. for 90 minutes, and hasa coefficient of humidity expansion of 15 ppm/% RH or less.
 16. Apolyimide film for a display device supporting base according to claim13 or 14, wherein the polyimide film forms the display device throughintermediation of a gas barrier layer, and has a difference incoefficient of thermal expansion from the gas barrier layer of 10 ppm/Kor less.
 17. A polyimide film for a display device supporting baseaccording to claim 13 or 14, wherein the polyimide film is used as asupporting base for an organic EL device comprising an organic ELlight-emitting layer.
 18. A polyimide film for a display devicesupporting base according to claim 13 or 14, wherein the polyimide filmis used for a display device comprising a color filter substrate, andthe supporting base comprising the polyimide film has a color filterlayer formed thereon.
 19. A polyimide film for a display devicesupporting base according to claim 13 or 14, wherein the polyimide filmcomprises a single polyimide layer or a plurality of polyimide layerscomprising a main polyimide layer containing polyimide having astructural unit represented by the following general formula (4) and astructural unit represented by the following general formula (5) at amolar ratio “(4):(5)” of from 50:50 to 100:0.


20. A manufacturing method for a display device, the manufacturingmethod comprising: applying a resin solution of polyimide or a polyimideprecursor onto a base substrate so that a thickness of a polyimide filmbecomes 50 μm or less; forming the polyimide film on the base substrateby completing heat treatment; forming a stress relaxation layer forpreventing stretching of the polyimide film on the polyimide film;removing the base substrate under a state in which the polyimide filmand the stress relaxation layer are laminated; and forming a member fora display device, the polyimide film comprising a single polyimide layeror a plurality of polyimide layers comprising a main polyimide layercontaining polyimide having 70 mol % or more of a structural unitrepresented by the following general formula (1):

General Formula in the formula (1), Ar₁ represents a tetravalent organicgroup having an aromatic ring, and Ar₂ represents a divalent organicgroup represented by the following general formula (2) or (3),

General Formula in the general formula (2) or the general formula (3),R₁ to R₈ each independently represent a hydrogen atom, a fluorine atom,an alkyl group or alkoxy group having 1 to 5 carbon atoms, or afluorine-substituted hydrocarbon group, and at least one of R₁ to R₄ inthe general formula (2) and at least one of R₁ to R₈ in the generalformula (3) each represent a fluorine atom or a fluorine-substitutedhydrocarbon group.
 21. A production method for a polyimide film for adisplay device supporting base, the production method comprising:applying a resin solution of polyimide or a polyimide precursor onto abase substrate so that a thickness of a polyimide film becomes 50 μm orless; forming the polyimide film on the base substrate by completingheat treatment; forming a stress relaxation layer for preventingstretching of the polyimide film on the polyimide film; and removing thebase substrate under a state in which the polyimide film and the stressrelaxation layer are laminated, the polyimide film comprising a singlepolyimide layer or a plurality of polyimide layers comprising a mainpolyimide layer containing polyimide having 70 mol % or more of astructural unit represented by the following general formula (1):

General Formula in the formula (1), Ar₁ represents a tetravalent organicgroup having an aromatic ring, and Ar₂ represents a divalent organicgroup represented by the following general formula (2) or (3):

General Formula in the general formula (2) or the general formula (3),R₁ to R₈ each independently represent a hydrogen atom, a fluorine atom,an alkyl group or alkoxy group having 1 to 5 carbon atoms, or afluorine-substituted hydrocarbon group, and at least one of R₁ to R₄ inthe general formula (2) and at least one of R₁ to R₈ in the generalformula (3) each represent a fluorine atom or a fluorine-substitutedhydrocarbon group.
 22. A production method for a polyimide film for adisplay device supporting base according to claim 21, wherein the basesubstrate comprises glass.